Changing background color according to variable IN HARLOWE

Hi, I saw there was a post about this for Sugarcube formats, but was wondering if it is possible in Harlowe: I'd like to change the background color of my passages in CSS according to the value of a variable (EG rgba(255,0,0,$variable) where $variable is between 0 and 1).

Is this possible in Harlowe? Or would I need to use JS? I'm new to Twine so appreciate any help.